Dispenser cathode



Jan. 6, 1959 H. J. LEMMENS ETAL DISPENSER CATHODE Filed Feb. 18. 1954INVENTORS v HENDRIKUS JOHANNES LEMMENS MARINUS JACObUS JANSEN PIETERWILLEM HAAYMAN BY i AGENT lPatent 2,867,742 .Patented Jan. `@1959 Y Y-DISPENSER v*(iA'TIroDE Application February 18, 1954, Serial No. 411,222

Claims priority, application Netherlands February 26, 1953 4 Claims.(Cl. 313-346) This invention relates to a dispenser cathode having acavity which contains a supply of alkaline-earth metal compounds, thewall of the cavity being constituted in part by a porous body of one ofthe refractory metals tungsten, molybdenum, tantalum, 'hafnium orniobium, which body has been sintered at a high temperature and thepores of which constitute the largest apertures in the wall of thecavity. The invention furthermore relates to a method of manufacturingsuch a cathode.

Dispenser cathodes of the above-mentioned kind are known for theirfavourable properties in regard to life and emission and also for theeasy workability of t-he emission surface into the desired shape. Thelling of the cavity in the dispenser cathode consists of carbonateswhich are disintegrated into oxides during the exhaustion of thedischarge tube.

More particularly for cathodes intended for working temperatures higherthan 1200a C. it is desirable that the porous body should have a lowporosity in order to limit the evaporation of barium. However, a lowporosity involves the disadvantage that the carbon dioxide developedduring the disintegration of the carbonates exhibits a comparativelyhigh pressure in the cavity and nevertheless escapes only slowly. At'thehigh temperatures occurring, the carbon dioxide can cause oxidation ofthe porous body or other parts of the wall of the cavity, so that theformation of free alkaline-earth metal may be impeded, whilstfurthermore other unpleasant phenomena may be involved.

The object of the invention is to provide a cathode which does notexhibit the above-mentioned disadvantages.

According to the invention, in a dispenser cathode having a cavity whichcontains a supply of alkaline-earth metal compounds and of which thewall is constituted n part by a porous body of one of the refractorymetals tungsten, molybdenum, ltantalum, hafnium or niobium, which bodyhas been sintered at a high temperature and the pores yof whichconstitute the largest apertures in the wall of the cavity, the supplyconsists of a compact mass of alkaline-earth oxide which has beensintered together.

The sintered compact mass of alkaline-earth oxide may be obtained bypressing alkaline-earth carbonate and subsequently heating it for sometime in a vacuum oven or in an indiierent atmosphere at a hightemperature. The heating may be effected either in a Crucible of anysuitable metal, for example platinum, or in the cavity of the dispensercathode itself before being closed by the porous body, which cavity isformed, for example, by

,a molybdenum body, but in this case it is desirable that .thecarbonates should be disintegrated at a comparatively low temperature,whichmaybe followed by sinter- '.''ing' togetheratfaJ-hig'heritemperature. rThe question as to whether it isnecessarytof--worky'in vacuo or'vin an findii'erentsfatmospheres is Y:dependent inter alia r upon the pressure that is used and the resultanttime required for compact sintering in connection with evaporation ofthe oxides.

The compact mass of alkaline-earth oxide which has been sinteredtogether is comparatively well resistant to air, so that it can `beworked in dispenser cathodes in the air without any special precautionsbeing necessary.

Dispenser cathodes comprising such a presintered mass of alkaline-earthoxide split off very little gas only during their preparation in thetube, since disintegration of the carbonates is not necessary and theoxide mass has been well predegassed due to the treatment at hightemperature.

In order that the invention may be readily carried into effect, it willnow be described with reference to the accompanying drawing showing, byway of example, two cathodes each comprising a supply according to theinvention.

ln Fig. l, reference numeral 1 indicates a molybdenum body, in which 2is the space intended for a filament and 3 is a cavity containing asupply of alkaline-earth oxide 4. Above the cavity there is shown aporous tungsten body 5 which, after the supply 4 has been provided, issecured by butt welding to a sharp edge 6. The supply 4 may be formed byeither of the two following methods. The first method is the following:a pressed pastille of barium strontium carbonate containing at least 25at.% of strontium carbonate is heated in a platinum crucible in a vacuumoven at 1500 C. for 5 minutes. TheA pas tille is subsequently providedin the cavity 3. The second method is the following: a quantity ofbarium strontium carbonate is pressed into the cavity 3, whereafter themolybdenum body containing the supply is at first heated at 1000 C. forl hour and subsequently at a temperature of 1500 C. for 5 minutes. Thedurability of the pastille obtained by the rst method as well as that ofthe open space with the supply provided therein is at least hours inair.

In Fig. 2, reference number 7 indicates a cylindrical molybdenum bodyhaving a at flange 8 which is engaged by a porous tungsten body 9, a atange 10 of a molybdenum cylinder 11 engaging the' other side of thetungsten body. The cavity between the body 9 and the cylindrical body 7contains a supply of alkaline-earth oxide, indicated by 12, which isobtained by precipitation in a compact form. The precipitation isfollowed by heating in the same manner as in the case of the pressedcarbonate shown in Fig. l. It is alternatively possible for a ymass ofalkaline-earth carbonate of the desired shape to be heated in a vacuumoven and provided around the body 7. If sufficient allowance is made forcontraction, it is possible to obtain a small cylinder of oxide of thecorrect size, or a plurality of sectors of a. cylinder, or it ispossible to provide a plurality of profiled rods between the bodies 7and 9. After the supply has been provided, the cathode is closed at 13,14 and 15 by means of a circular welding seam.

What is claimed is:

l. In the method of manufacturing a dispenser cathode comprising a bodyof refractory metal having an internal cavity and at least one emissivesurface portion, one wall portion surrounding said cavity being porous,the pores of said latter wall portion constituting the largestpassageways connecting the cavity to said surface, the steps of forminga mixture of alkaline earth carbonates, sin- 25% of strontiumcarbonate.'` s

3. The method of claim 1 in which the sintered body is formed outside ofthe cavity.

4. The method of claim 1 in which the sintered body is formed within'thez'avity but before" the"V cavityV is sealed.

References Cited in the ie of this patent UNITED STATES PATENTS FrancisFeb. 23, 1937 Gustin Aug. 29, 1939 Gaidieset al. Oct. 10. 1939Germeshausen Dec. 27, 1949 Lemmens et al Feb. 27, 1951 Levi Oct. 25,1955

1. IN THE METHOD OF MANUFACTURING A DISPENSER CATHODE COMPRISING A BODYOF REFRACTORY METAL HAVING AN INTERNAL CAVITY AND AT LEAST ONE EMISSIVESURFACE PORTION, ONE WALL PORTION SURROUNDING SAID CAVITY BEING POROUS,THE PORES OF SAID LATTER WALL PORTION CONSTITUTING THE LARGESTPASSAGEWAYS CONNECTING THE CAVITY TO SAID SURFACE, THE STEPS OF FORMINGA MIXTURE OF ALKALINE EARTH CARBONATES, SINTERING SAID MIXTURE IN ANON-REACTIVE ATMOSPHERE TO CONVERT THE SAME TO A COHERENT BODY OFALKALINE EARTH OXIDES SUBSTANTIALLY IMPERVIOUS TO REACTIVE GASES, ANDCLOSING SAID CAVITY AFTER PLACING THE SINTERED BODY THEREIN WHEREBY SAIDCATHODE CAN BE ACTIVATED SUBSTANTIALLY WITHOUT THE RELEASE OF GASES.